Many fluidized bed reactors contain a mass of fine particles. In the lower portion of the reactor a dispersion plate is provided having a plurality of injection nozzles through which fluidizing gas is distributed from a windbox into the reactor. The gas (such as air) introduced through the nozzles fluidizes the mass of fine particles. Such fluidized bed reactors are typically utilized in a variety of different processes such as combustion, gasification, and heat transfer processes. Depending upon the particular processes, sand, limestone, coal, ash, or pieces of refractory material broken off from the reactor wall, are fluidized. This results in a severe environment for the injection nozzles, subjecting them to corrosion and erosion.
In conventional fluidized bed reactors, especially under low load conditions, particles tend to build up near the injection nozzles and adversely affect the air distribution provided by the nozzles. Fine particles in the mass of particles interfere with the air supplied through the nozzles and can cause plugging of the air openings. Further, there is a tendency for particles to back flow through the nozzles into the windbox (air chamber). This occurs when the supply of air through the injection nozzles is stopped, resulting in the pressure difference between the reactor chamber and the windbox under the distribution plate to reverse. The pressure above the distribution plate grow higher than the pressure in the windbox beneath the distribution plate and consequently the fine particles of the particle mass tend to flow backward through the nozzles into the windbox.
Backflow tendencies also cause problems under steady state running conditions. The pressure in the fluidized bed reactor is always pulsating and can momentarily and locally decrease enough to cause backflow. Then at low load conditions, the tendency grows as the pressure difference between both sides of the distribution plate becomes smaller.
The backflow of particles through the nozzle is especially a problem in circulating fluidized beds when the bed consists of fine particles fluidized at high flow rates, entrained from the reactor and recycled after gas separation.
Not only does backflow cause problems in the windbox, the fine particles flowing back into the nozzles often move back and forth within the nozzles as a result of pulsations in the reactor. This back and forth flow of particles in the nozzles, through the openings in the nozzles, causes the nozzles to wear out prematurely.
The backflow problem can be avoided by keeping the pressure difference sufficiently high by increasing the flow of fluidizing gas pursuant to the formula dp=k V.sup.2 /2, which indicates that the pressure different (dp) is dependent on gas flow V and gas density, the value k equal to the nozzle constant. However due to high power costs, it is not feasible to eliminate backflow by increasing the pressure difference.
While the problem of backflowing particles in fluidized beds is well known, and many solutions have been proposed, such solutions have not been entirely successful. It is believed that an optimum design which provides sufficient pressure drop to fluidize the bed evenly yet still allows for operation at low load without backflow has not yet been provided.
According to the present invention, injection nozzles for a fluidized bed reactor are provided which overcome the above-mentioned problems, and thus allow a substantially even bed fluidization while still allowing for operation at low loads without backflow. The nozzles according to the present invention are better suited to the corrosive and erosive conditions normally existing in fluidizing beds, and are less sensitive to mechanical shock.
According to the broadest aspect of the present invention, a fluidized bed reactor is provided containing a mass of fine particles, and in its lower part a dispersion plate having a plurality of injection nozzles through which fluidizing gas is distributed from a windbox into the reactor. At least some of the injection nozzles comprise an upper part, means defining at least one opening, and solid means. The upper part is of high heat and wear resistant material adapted to contact the mass of particles. The means defining at least one opening in each nozzle allows passage of fluidizing gas through the opening(s) between the upper part of the nozzle and the windbow. Solid means substantially abut the means for defining the openings for allowing passage of gas therethrough but preventing passage of the fine particles therethrough. The solid means preferably comprise a sintered metal. Alternatively the solid means may comprise a metal wire mesh acid proof filter, a porous ceramic material, or a high temperature ceramic filter. For example a pair of concentric adjacent porous ceramic tubes may be provided with different effective gas permeabilities.
A solid walled stand pipe may extend upwardly from the windbox with the openings above the stand pipe and directed downwardly outwardly. The stand pipe may be disposed interiorly of the upper part with the solid means supported by the stand pipe and abutting the upper part. The stand pipe may support a horizontal solid metal ring having the openings therein with the solid means sandwiched between the upper part and the ring. The solid means may comprise a tubular element having a generally vertical axis, and the solid means may have pores with a pore size of between about 50-1,000 micrometers. Where the solid means comprises sintered metal, it is preferably in the form of a tube disposed between and abutting the stand pipe and the upper part.
It is the primary object of the present invention to provide for even distribution of fluidizing gas while preventing the backflow of solids into the gas injection nozzles, in a fluidized bed reactor. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.